Median nerve evoked responses and explicit memory during recovery from isoflurane/nitrous oxide anesthesia

To evaluate median nerve somatosensory evoked responses during recovery from anesthesia in relation to clinical findings.

Twenty-two gynecologic patients received isoflurane in nitrous oxide for anesthesia. Midlatency somatosensory evoked responses (N20, P25, N35, P45, N50) were recorded the day before surgery (AWAKE), during steady state anesthesia (STABLE), and every five minutes after discontinuation of anesthesia until the patients were able to name a shown object correctly (RECOVERY). Next day the patients were questioned with a structured interview about their explicit memory of the immediate recovery period and classified into groups: No-MEM (no memory) and MEM (memory). Multivariate analysis of variance compared electrophysiological parameters at the different time points and between the two memory groups.

During STABLE isoflurane/N2O anesthesia, all cortical amplitudes were reduced (P< or =0.003) and all latencies were prolonged compared with AWAKE (P<0.001). At RECOVERY the latencies N35, P45, N50 remained prolonged (P< or =0.001), while the amplitudes N20P25 and P45N50 were reduced in comparison to AWAKE (P< or =0.02). The latencies P45 (48+/-8 vs. 61+/-9 msec) and N50 (67+/-12 vs. 81+/-10 msec) were shorter in the patients of the group MEM (P< or =0.03) at RECOVERY.

The reversibility of anesthetic induced changes in amplitudes and latencies of median nerve somatosensory evoked responses reflected clinical awakening during emergence from isoflurane/nitrous oxide anesthesia. In the patients who had recall for the immediate recovery period, the reversibility of anesthetic induced changes of components P45 and N50 was faster than in patients without recall.